Yes, by making the UPLINK FM receiver have a widebandwidth of say 40 KHz,
then even +/- 10 KHz of normal UHF doppler stays in the passband. THus
the recovered audio is unaffected by uplink doppler. THis audio then
simply goes into a normal 3 KHz SSB downlink.
Again, this is only a quick-demo way of getting a low doppler multi-stream
PSK-31 downlink to "test the waters". THe ultimate objective for the
"next" satellite would be the INBAND-INVERTING transponder so that then
you can have 20 independent uplink QSO's... with only 45 HZ total
doppler...
I have posted the design of the INBAND-INVERTING transponder on
http://web.usna.navy.mil/~bruninga/photos/PSKSAT.jpg
Problem is, I havent figured out how much isolation I can get in such a
tiny space... yet...
bob
On Sun, 25 Feb 2001 ka1lm@amsat.org wrote:
> On Sun, 25 Feb 2001, MCGWIER ROBERT wrote:
>
> > You have forgotten the contribution to Doppler caused by the uplink.
> > Even with an inverting transponder, and practically no frequency
> > tracking in most of the PSK-31 implementations <<that I know about>>,
> > this will be a difficulty.
>
> Bob:
>
> Sticking my neck out, as I have *NO* experience designing receivers or
> other flight hardware...
>
> WB4APR said:
>
> > > On this short fuse experiment, we would simply put 20 PSK-31 audio
> > > streams into a single UHF FM uplink having an onboard wideband (no
> > > doppler) receiver. THus, the only doppler is the small 600 Hz
> > > experienced on the 10m SSB downlink. The PSK-31 software on the ground
> > > can EASILY track this, since ALL QSO's move together!
>
> I think WB4APR Bob's statement about doppler shift is correct *if* the UHF
> uplink receiver is designed as follows:
>
> The receiver is FM. Use the discriminator output to drive an AFC circuit.
> Assume the UHF receiver and AFC circuit has sufficient bandwidth to
> accomodate the audio stream and +/- 9 KHz of doppler shift. Assume the
> uplink has sufficient ERP to overcome the greater noise in an uplink wide
> enough to meet this requirement (say 20-30 KHz total). Convert the signal
> to baseband audio before being fed back into the transmitter.
>
> I think this gives us a no-tune uplink. The uplink receiver is handling
> doppler tuning via AFC.
>
> I recall that the AO-27 uplink uses this approach. Doppler shift on 145
> MHz is +/-3 KHz, so the total receiver bandwidth can be smaller and
> therefore provide a better SNR. I **think** (8+ years is a long time) the
> Microsat uplinks use the same approach, albeit again on 145 MHz instead of
> 435 MHz.
>
> Bob (WB4APR), how do you intend to "process" the uplink - audio bent pipe?
> It seems to me that other than a wider receiver bandwidth due to doppler
> and therefore greater noise in the uplink, the approach is sound. What do
> you expect the bandwidth to be of the combined uplink signal? (no
> experience with PSK31 here.)
>
> 73 Steve KA1LM
>
> -------------------------------------------------------------------------
> Stephan A. Greene sgreene@patriot.net
> Amateur KA1LM@amsat.org Grid FM18hx 38 59'83.33"N 77 23'6.15"W
> -------------------------------------------------------------------------
>
>
de WB4APR@amsat.org, Bob
APRS LIVE pages http://web.usna.navy.mil/~bruninga/aprs.html
APRS SATELLITES http://web.usna.navy.mil/~bruninga/astars.html
MIM/Mic-E/Mic-Lite http://www.toad.net/~wclement/bruninga/mic-lite.html
----
Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org